Urea. This compound had long been known as a constituent of the urine not only of mammals, but also of reptiles and birds, and had been early obtained from human urine. In 1828, however, it was synthesized by Wohler,who showed it to be prepared from purely inorganic compounds. This being the first artificial preparation of a so-called organic compound, was the first of the now very numerous preparations which demonstrate that between the organic and inorganic compounds there is no essential difference. To prepare urea from urine the liquid is evaporated to a syrupy consistency, and nitric acid is added, by which a salt of urea (urea nitrate) is precipitated. This is purified by recrystallizing, and from it the urea itself is easily obtained. The artificial preparation is most easily effected by Wohler's original method, i.e. by heating ammonium iso-cyanate, when a molecular transformation takes place, and urea is formed. It is a white crystalline compound, which forms rhombic needles, with a taste somewhat like that of saltpetre. It is very soluble in water and alcohol, but nearly insoluble in ether. It acts as a strong organic base, uniting with acids to form salts. Its solution, exposed to air, slowly decomposes, forming carbonic acid and ammonia, the same decomposition occurring with urine. It yields a large number of derivatives by substitution of hydro-carbon radicals for the hydrogen of urea, many of these compounds being important physiological substances. The quantitative determination of urea in urine is a matter of considerable importance in medicine, and for it several methods are available.